1. Field of the Invention
The present invention relates to a field effect transistor, and more particularly, to a switching field effect transistor (FET) using abrupt metal-insulator transition.
2. Description of the Related Art
Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) have been widely used as micro and super-speed switching transistors. A MOSFET has two pn junction structures showing linear characteristics at a low drain voltage as a basic structure. However, when a channel length is reduced to about 50 nm or below as the degree of integration of a device increases, an increase in a depletion layer changes the concentration of a carrier and an decrease of the depth of the gate insulator remarkably makes current flowing between a gate and a channel.
To overcome this problem, IBM institute has performed Mott FETs using the Mott-Hubbard insulator in a channel layer in reference “D. M. Newns, J. A. Misewich, C. C. Tsuei, A. Gupta, B. A. Scott, and A. Schrott, Appl. Phys. Lett. 73, 780 (1998)”. The Mott-Hubbard insulator undergoes a transition from an antiferromagnetic insulator to an metal. This transition is called the Mott-Hubbard metal-insulator transition in reference “J. Hubbard, Proc. Roy. Sci. (London) A276, 238 (1963), A281, 401 (1963)”. This is a continuous (or second order) phase transition. Unlike MOSFETs, Mott FETs perform an ON/OFF operation according to metal-insulator transition and do not have a depletion layer, thereby remarkably improving the degree of integration of a device and achieving a higher-speed switching characteristic than MOSFETs.
Since Mott-Hubbard FETs use continuous metal-insulator transition, charges used as carriers should be continuously added until the best metallic characteristics reach. Accordingly, the added charges must have a high concentration. Generally, charges N per unit area can be expressed by Equation (1).
                    N        =                              ɛ            ed                    ⁢                      V            g                                              (        1        )            
Here, “∈” denotes the dielectric constant of a gate insulator, “e” denotes a basic charge, “d,” denotes the thickness of the gate insulator, and “Vg” denotes a gate voltage.
For example, in the case of La2CuO4, which is one of the materials falling under the group Mott-Hubbard insulator, when holes are added to La2CuO4, the characteristics of La2-xSrxCuO4(LSCO) appear, and a metal having best hole carriers at x=0.15 (15%) is obtained. Here, the added holes become carriers. Generally, x=0.15 is a high concentration, so if the N value increases, the dielectric constant of the gate insulator increases, the thickness of the gate insulator decreases, or the gate voltage increases. However, when the dielectric constant is too great, the fatigue characteristics of a dielectric sharply worsens during a high-speed switching operation, thereby reducing the life of a transistor. Moreover, there is a limit in decreasing the thickness of the gate insulator due to limitations in fabrication processes. In addition, when the gate voltage increases, power consumption also increases, which makes it difficult to be the transistor with a low power.